Radio frequency channel coordination involves plans and procedures followed by users of a common band of radio frequencies to minimize and control potential conflict and interference between systems. A key aspect of the procedure involves cooperative radio frequency planning. Because there are so many variables involved with frequency coordination and planning, even with good cooperation, mistakes can occur that impact other users, or there can be intentional channel misuse. Historically, there has been a high occurrence of these kinds of problems at large events.
Radio frequency systems must be designed in such a manner as to minimize frequency allocation issues and to negate objectionable interference with other existing or planned systems using the same or affected frequency bands. This coordination is facilitated by sharing coordination data among users, so that accurate and up-to-date information is available with which estimates of potential interference can be made, even during the design stage of a temporary system. Radio frequency interference studies and frequency coordination are necessary not only when designing a new system, but also when the potential interference effects of other users' radio transmission proposals on existing and planned temporary systems is considered. Thus, coordination is needed when one party develops and executes transmission plans as well as when reacting to other parties' plans.
The purpose of frequency coordination is to predict and control potential interference between devices. Radio frequency interference is caused by the presence of a non-desired interfering signal in a channel being used by a desired signal (linear or co-channel interference), as well as by the mixing of one or more non-desired interfering signals with the desired signal resulting in intermodulation products and added noise in the interfered system (non-linear interference). If not controlled to acceptable levels, interference can seriously degrade system usability, reliability and noise performance, thus rendering an important channel anywhere from noisy to useless.
The specific effects of interference to a given system are dependent on many factors, including but not limited to:
the modulation characteristics of the desired and interfering signals,
the bandwidths of the signals,
the mathematical relationship (such as intermodulation (IM) characteristics) between the two signals,
the extent of channel overlap or frequency separation between the two signals,
the signal strengths of the two signals,
the characteristics of the transmitters receivers, and antennas, and
the relative level of the interference signal with respect to the normal noise level of the interfered system.
The effect of radio frequency interference varies not only with the type of interfering signal, but also with the nature of the interfered (desired) signal. For example, in analog systems, voice signal interference into another voice channel generally results in crosstalk. The effects of this interference generally increase with the voice circuit loading on both the interfering and interfered channels. Interference to data signals may cause bit errors, with the effects of the interference more severe for more complex digital modulation schemes. Interference is most severe when the interfering channel shares the same frequency slot as the interfered channel, a situation referred to as “co-channel interference.” Because channel filters are not perfect, interference can also occur between channels that do not overlap, especially in temporary situations. This “adjacent channel” interference may occur when channels are directly adjacent in frequency or, in some cases, when they are separated by an amount equivalent to several channel bandwidths.
The amount or level of noise resulting from interference is generally related to the received levels of the desired signal and the interfering signal. The ratio of these signals, referred to as the “carrier-to-interference” ratio (C/I), may be considered a radio frequency version of signal-to-noise ratio (S/N). In fact, C/I is directly related to baseband S/N in a multichannel analog system. High C/I values correspond to high S/N values. Since circuit noise levels are inversely proportional to S/N, it follows that high C/I values correspond to low interference noise levels. Another and perhaps more direct way of considering interference is to compare it to some absolute level of noise in the system, rather than dealing strictly in ratios. Both ratios and absolute levels are used in different situations to control interference.
Several parts of the FCC Rules and Regulations contain requirements and guidelines concerning prior coordination. Part 2 of the FCC Rules contains the table of FCC frequency allocations as well as definitions of various types of emissions and methods of calculating signal bandwidths. Besides satisfying an FCC requirement, frequency coordination with other radio users is conducted basically for two reasons: (1) to determine if a system is technically feasible and acceptable to other users from an interference standpoint; and (2) to provide interference protection for the proposed system until applications are filed with the FCC. The FCC Rules generally state that coordination involves two elements: notification and response. (Section 21.100(d) of the FCC Rules contains the basic procedural guidelines for coordination.)
For permanent systems, once preliminary design of a new radio system is complete, frequency coordination involves the following: (1) distribution of a prior coordination notification to all parties who could be affected by the new proposal; (2) analyses by those parties of potential interference (both caused by and caused to the proposed system); and (3) responses by those parties, generally stating agreement or objection to the proposal on the basis of their analyses of potential interference. If all parties agree, the coordination process is considered complete and the proposed system is given interference protection. Note that prior coordination does not provide absolute assurance that interference will not occur, because of the many variables involved.
If certain parties object to the proposed system because of anticipated interference problems, the originating coordinator may find the problems insurmountable and terminate the proposal, or they may be able to modify the proposal to resolve the problems. If modifications are possible, a second coordination notification is sent to all parties to determine if the proposed system, as modified, is acceptable. Sometimes a proposed system may be acceptable to a given party as initially coordinated, but subsequent modifications make it unacceptable. The process of modification and subsequent notification and response may continue for some time until the proposal becomes acceptable to all concerned parties.
One situation that must be considered is the one involving a successfully coordinated temporary system that, when constructed, causes harmful interference that was not anticipated. When this type of problem occurs, the Commission initially encourages the parties involved to attempt to clear the problem jointly, for example, by changing antenna heights, frequencies, or signal polarizations, if possible. If all reasonable efforts prove unsuccessful, the FCC may rely on the “first come, first served” principle and require the “last one in” to correct the problem by any means necessary, including terminating operation.
Since its establishment of frequency coordination requirements and guidelines, the FCC has assumed an essentially passive role in day-to-day coordination and enforcement. The Commission staff is occasionally contacted to give its view on specific problems and procedures and to provide interpretations of the Rules. Day-to-day interaction among frequency coordinators is generally in accordance with an industry practice that has developed within the FCC's coordination guidelines.
Because of the complex nature of temporary frequency coordination and allocation, it is therefore useful to have a technology that can assist with or even automatically perform these functions for wireless device users.